1. Field of the Invention
The present invention is a system for automatically and periodically updating and adjusting an on-line pressurized water reactor core analytical model, which consists of a data file of parameters that describe the reactor core, to insure that at all times the model (data file) closely matches the then current characteristics of the modeled and monitored core, so that real-time and anticipating graphics displays representative of the operating characteristics of the actual core can be generated for a plant operator's use and an attached core parameter predictor can be reliably initialized by a user at any time, even through only minimal core monitoring and analytical capabilities are available.
2. Description of the Related Art
An analytical core model has, in the context of the present invention, three essential components. The first of these is the broad collection of facts that encompasses the physical description of the core and the nuclear cross section data sets that describe the relative rates at which various nuclear reactions will occur in the core. The second of these several components is the current set of spatial distributions of time varying concentrations of certain transient nuclear isotopes that significantly affect local neutron balances throughout the core. Typical isotopes of concern are xenon-135 and samarium-149 precursors, iodine-135 and promethium-149, and, on a longer time scale, long term burn-up. The last essential component of the model is that small set of coefficients that, coupled with certain algorithms embedded in neutronics calculation sequences, allows complex nuclear phenomenom that are known to be operative in the reactor core to be replicated with a sufficient degree of accuracy by very simple approximation.
The concept of updating the analytical core model relates to the tracking in time of the changes that occur in the local concentrations of the several nuclides that are of primary consideration in satisfying the second component of the analytical core model. The concept of adjusting the analytical core model relates to modifying one or more of the co-efficients that make up the third component of the analytical core model, so that the simple approximations used to replicate the affects of complex nuclear processes in the core provide the best available replication. With respect to the above components, two distinct problems pose themselves.
The first problem relates to obtaining, on-line, sufficient information regarding the current distribution of nuclear power, iodine-135 and xenon-135 to be able to supply the reactor operator with reliable, concise indicators both of actual current core conditions and of trends in current core conditions, so that the operator can effectively and efficiently exercise control functions using xenon distribution displays as described in U.S. Pat. No. 4,642,213. In this context, the reactor operator can well include dedicated automatic control systems that carry out nominally human control functions. Various known approaches to successfully solving this problem include the use of the responses of many strings of fixed incore detectors to synthesize full three dimensional core power distributions from which the needed indicators are readily extracted, the use of on-line three dimensional analytical core models, augmented by a modest number of plant instrumentation signals, again to generate a comparably useable three dimensional core power distribution, and the use only of the signals from conventional plant instrumentation, filtered through empirically derived correlations, to produce detailed one dimensional core average axial power distributions and the needed derivative indicators such as is described in the U.S. Pat. No. 4,774,050. The use of many strings of fixed incore detectors necessarily commits the plant owner to relatively high initial and ongoing equipment costs. The use of three dimensional analytical models is highly computer resource intensive. The use of purely empirical correlations requires frequent careful calibration of the plant instrumentation and of the correlations themselves.
The second problem relates to insuring that the analytical core model be sufficiently well matched to the operating characteristics of the corresponding reactor core, so that the predictions of core behavior remain stable and realistic over periods of tens of hours in the future, granted a valid initialization of a core predictor. Attempts have been made in the past using conventional core models to track certain operating parameters (power level, control bank position, etc.) in pressurized water reactors as plant operations proceed and to periodically update the analytical core models by, in affect, making projections of core response as the core actually responds. In none of these cases was an attempt made to adjust online any of the set of coefficients in the third component of the core model to force the model to match the actual nuclear characteristics of the core. In all such cases in which the reactor core involved was unstable or nearly unstable to spatial xenon oscillations, deviations between the calculated core average axial power distribution and the actual measured core average axial power distribution, as inferred by comparison of calculated axial offset with measured offset, for example, set in quickly and grew rapidly to a point where the current analytical results were useless and any subsequent predictions of core response would be, at best, suspect. The recently demonstrated use of a full, three dimensional nodal core model, that is at least weakly coupled to the plant instrumentation and that has provision for periodically adjusting the actual neutronics characteristics of the model to force reasonable agreement of the calculated core characteristics for the measurable values of those characteristics, offers the only currently known avenue that could lead to solving this second problem, albeit at a rather high commitment of computer resources. In the face of the foregoing, it is evident that what is needed is a simple on-line, one-dimensional analytical core model, which is far less computer resource intensive than a full three dimensional analytical core model, that can utilize the responses of conventional reactor instrumentation both to periodically update the model data file to account for ongoing plant operations and to, when deemed necessary, adjust the actual neutronics characteristics of the core model to ensure that calculated axial power distributions continue to closely track measured axial power distributions, as indicated by comparing calculated and measured values of axial offset and axial pinch. The close match of the core model to the actual reactor obtained by utilizing monitored reactor instrumentation responses both to continuously update the axial power, iodine, xenon, promethium, samarium and long term burn-up distributions in the core model and to concurrently adjust the nuclear characteristics of the model to match the nuclear characteristics of the core then provides a relatively inexpensive, readily implemented method for solving both of the problems identified above.
U.S. Pat. No. 4,711,753 describes a scheme for utilizing the results obtained from an equilibrium full core flux map to calibrate or adjust certain elements of the analytical model (or data filed) to be used by a core response predictor. In particular, the axial distribution of the transverse buckling values, B.sup.2.sub.xy (Z) is adjusted, so that the calculated axial power distribution in the core model closely approximates the core average axial power distribution derived from the flux map. The constraint that the flux map be taken under stable equilibrium core conditions is imposed because no information regarding transient iodine, xenon promethium or samarium distributions can be derived from a single flux map. In this approach the axial distribution of the transverse buckling values takes the form: ##EQU1## which when expanded becomes: EQU B.sup.2.sub.xy (Z)=A.sub.1 F.sub.1 (Z)+A.sub.4 F.sub.4 (Z)+A.sub.5 F.sub.5 (Z)+. . . (2)
The calibration or adjustment process consists in determining the values of the set of expansion coefficients, A.sub.n, that results in the best matching of a series of integral parameters characterizing the calculated axial power distributions to the corresponding parameters characterizing the core average axial power distribution derived from the flux map. The particular parameters include the well known axial offset parameter (AO) and other progressively higher order terms involving integrals over thirds, quarters and fifths of the core height. Due to the complex relationship of calculated axial power distribution to the core model, of which transverse buckling is only one of several components, the values of the expansion coefficients must be found by a guided trial and error search process involving several nested levels of search, details of which are given in the referenced patent. The whole procedure is feasible only because the particular set of expansion functions used, the F.sub.n (Z) functions, has the unique property of effectively decoupling the searches for the successive expansion coefficient values. Thus, the A.sub.1 coefficient influences the reactivity balance, but does not significantly affect any aspect of power distribution. The A.sub.2 coefficient controls the axial offset aspect of the power distribution but does not materially affect axial pinch (AP), etc. aspects, and so on. That the decoupling is effective has been demonstrated theoretically, and by repeated application of the calibration procedure to a variety of analytical core models. However, the calibration procedure described in U.S. Pat. No. 4,711,753 is applicable only when the results of a recent flux map taken under equilibrium core conditions are available. This occurs typically once a month in a conventional operating pressurized water reactor. Therefore, the needed system must be able to readjust the values of at least the dominant transverse buckling coefficients, specifically the A.sub.2 and A.sub.3 coefficients of equation (2) on-line and on a nominally continuous basis until the next monthly calibration becomes available to compensate for minor defects in the analytical core model and/or the neutronics algorithms being used.